The present invention relates to a method of treating a material, enabling the material to be caused to pass from one phase to a more ordered phase. For example, the present invention applies to imparting chemical order to magnetic alloys, in order to enhance their magnetic anisotropy.
Numerous phase transition methods are already known whereby a material is caused to pass from one phase to a more ordered phase by applying heat treatment. Numerous methods are also known that enable ordered phases of a material to be obtained directly at high temperature.
In particular, such methods are known that are applied to synthesizing magnetic alloys deposited on substrates.
Thus, certain magnetic alloys of the L10 type, such as Fe50 Pt50, Fe50 Pd50 and Co50 Pd50 type have high magnetic anisotropy and therefore present a particular advantage in applications where they are used for magnetic recording (K. R. Coffey, M. A. Parker, and J. K. Howard, IEE Trans. Magn., nxc2x031,. P.2737 (1995)). Such alloys present magnetic anisotropy that is twenty times greater than that of the cobalt-based alloys that are presently being used for computer hard disks. These alloys have periodically alternating planes each made up of atoms of a single species of the alloy. The ordered phase of these alloys, i.e. the phase having the highest magnetic anisotropy, can be obtained only by methods that require the use of high temperatures for synthesis. For example, such alloys were so far obtained:
either by epitaxial growth on monocrystalline substrates at a temperature of 500xc2x0 C. by cathode sputtering or by molecular beam epitaxy (R. F. C. Farrow, D. Weller, R. F. Marks, M. F. Toney, A. Cebollada, and G. R. Harp, J. Appl. Phys., nxc2x079, p. 5967 (1996); A. Cebollada, D. Weller, J. Sticht, G. R. Harp, R. F. C. Farrow, R. Marks, R. Savoy, and J. C. Scott, Phys. Rev. B, nxc2x050, p.3419 (1994), V. Gehanno, A. Marty, B. Gilles, and Y. Samson, Phys. Rev. B, nxc2x055, P.12552 (1997), P. Caro, A. Cebollada, D. Ravelosona, F. Briones, D. Garcia, M. Vasquez, A. Hernando, J. Appl. Phys., nxc2x081, p.5050 (1997); M. Visokay and R. Sinclair, Appl. Phys. Lett., nxc2x066, p.1692 (1995));
or by thermally annealing films previously deposited on monocrystalline substrates by cathode sputtering or by molecular beam epitaxy, the films being annealed at temperatures greater than 700xc2x0 C. (M. R. Visokay, B. M. Lairson, B. M. Clemens, R. Sinclair, J. Magn. Soc. Japan nxc2x019, p.399 (1995));
or else by thermally annealing disordered granular films of grain size less than 10 nm as previously deposited on amorphous substrates, e.g. of the SiO2 type by cathode sputtering or by molecular beam epitaxy, the films being annealed at temperatures greater than 600xc2x0 C.
In any event, the use of a high growth temperature, in particular a temperature greater than about 350xc2x0 C., is a major drawback in an industrial application since in addition to being relatively complex and expensive in energy terms, it can give rise to irreversible modifications of the substrate used for the recording medium. Furthermore, annealing disordered granular films does not enable ordered films to be obtained having uniform magnetic anisotropy. The orientation of the grains in such materials corresponds to a relatively broad distribution.
An object of the invention is to provide a method of treating a material enabling said material to be caused to pass from one phase to a more ordered phase, while avoiding the use of high temperatures.
According to the invention, this object is achieved by a method of treating a material to cause it to pass from one phase to a more ordered phase, the method comprising an operation of irradiating the material by low energy ions having an energy of the order one or two hundreds keV, the irradiation by said low energy ions inducing displacements of the atoms in the material towards positions that favor ordering of the material.
It is hereby recalled that light ions are ions having a mass below or equal to 16 atomic mass units.
By the method of the invention, certain atoms of the material are caused to be displaced, thereby enabling local rearrangement of the atomic array in a way that tends to decrease the free energy of the material. This rearrangement corresponds in particular to favoring chemical order in the material. The irradiation performs a function somewhat similar to that of a high temperature heat treatment, by means of the energy supplied by the irradiating particles, the difference being thatxe2x80x94as opposed to thermal treatmentxe2x80x94the order induced by irradiation is initiated by the low energy transfers due to atomic collisions.
Advantageously, the material is heated during its irradiation. The heating allows to control the atomic rearrangement towards quasi-equilibrium atomic positions. The heating favours atomic mobilities but do not replace collisions of the irradiating particles with the atomic lattice of the material.
The method of the invention advantageously includes the following characteristics, taken independently or in combination:
prior to the irradiation operation, the method includes an operation of depositing a thin layer of material on a substrate;
the material is an alloy or a compound; the alloy is advantageously a binary alloy whose most ordered phase is essentially made up of a stack of planes of atoms each comprising a majority of one of the component species of the alloy;
the material is magnetic;
the material is an alloy of iron and platinum, or cobalt and platinum, or ion and palladium;
the material is heated at a temperature of 350xc2x0 C. or less during the irradiation , advantageously this temperature is comprised between room temperature and 350xc2x0 C.;
the irradiating particles are helium ions;
the material is bombarded with an irradiating particle density of 5.1015 to 4xc3x971016 particles per square centimeter;
the material to be ordered by the irradiation operation is initially in a disordered or partially ordered state;
the material to be ordered is underlying other layers of material;
several superimposed layers are ordered simultaneously;
the irradiating particles bombard the material through a mask.
An advantageous application of the invention lies in the field of ultra-high density magnetic recording. The extraordinary development of multimedia technologies and services over the last few years has resulted in a race to increase recording density. The main limit on present magnetic recording techniques appears to be the xe2x80x9cparamagnetic limitxe2x80x9d, i.e. the size beneath which recorded bits fade away by a thermal effect. Several ways have been proposed for pushing back that limit. Thus, it has recently been proposed to replace the present recording medium material by discrete materials in which magnetic anisotropy boundaries are defined geometrically by lithographic methods. Nevertheless, the various techniques proposed on those lines require major changes to present hard disk technology. Another technique that has been considered for mitigating the fundamental paramagnetic limit is to use new materials having stronger magnetic anisotropy. This makes it possible to use grains of smaller size. This alternative appears to be more promising in the short term since it does not require major changes to present magnetic media technology. By means of the invention, a method is made available for chemically ordering a magnetic alloy at a temperature that is relatively low. By means of the invention, the imparting of chemical order is favored in alloys of the Fe50 Pt50, Fe50 Pd50 and Co50 Pd50 type. By means of this ordering, magnetic anisotropy is increased and magnetic recording materials are obtained for storing information at very high density. These materials have thermally stable grain sizes down to 3 nm, thereby making it possible to obtain storage densities of the order of tera bits per square centimeter (whereas present materials are limited to 100 giga bits per square centimeter).
In another aspect, the invention provides apparatus for magnetically recording information, the apparatus comprising a material deposited on a substrate at a temperature of less than 350xc2x0 C. and subjected to irradiation with irradiating particles suitable, by their nature and by their energy, for inducing displacements of the atoms in the material towards positions that favor relaxation of the material.